I'd probably say Final Fantasy II. Unpopular opinion I know, however the reason why I say this is because I played the GBA version and thought it was like Star Wars. It is a fun RPG that is kind of still bashed to this day. I can sort of see why though.
I have not seen a single real NES RPG posted yet. Mother and FF2 are both Famicom, not NES, and Zelda is not really an RPG.
In terms of NES RPGs, Dragon Warrior 3 is my favorite turn-based, and Crystalis is my favorite action RPG. Most of the good RPGs for the system were not released outside Japan though, so they are Famicom-exclusive.